1. Field of the Invention
This invention relates to an electronic musical instrument which synthesizes musical sounds, using logic orthogonal functions stored in digital form.
2. Description of the Prior Art
Typical examples of conventional electronic musical instruments employing the digital techniques are set forth in U.S. Pat. Nos. 3,515,792 and 3,809,786, in which their advantages are also referred to in brief as compared with prior art electronic musical instruments using the analog techniques.
In the abovesaid U.S. Pat. No. 3,515,792, amplitude values of the waveshape of a pipe organ tone or other instrument sound are quantized digitally at a plurality of sample points and stored in a fixed memory and the memory is repetitively read out at a rate determined by the note selected on the keyboard to obtain a digital representation of the selected musical note. After attack, release and other tone amplitude modulation effects are provided digitally, the musical note is converted to analog form for input to a sound system.
Further, for polyphonic tone synthesis, this system is constructed to use the abovesaid memory on a time shared basis to produce musical sounds selected by a plurality of keys.
The most serious defect of this system is the difficulty in producing a change of the spectral construction of a waveshape with the lapse of time which is desirable in terms of hearing sensation. In other words, the musical notes generated are limited only to the waveshapes read out of the fixed memory, and accordingly it is impossible to obtain a wide variety of tonal changes of ordinary instrument sounds having ever-changing tones.
The system disclosed in the other U.S. Pat. No. 3,809,786 solves many of the problems encountered in the abovesaid system. The basic structure of this system is to represent in digital form stored harmonic coefficients and a frequency selected by a key; to individually calculate Fourier components of respective moments in real time while establishing the waveshape period of the selected musical note; to sum them to digitally calculate the amplitude of the musical note of a specified time; and to reproduce the musical note to a sound.
That is, the use of a method of producing musical note by the employment of a Fourier algorithm not only settles the problems of the aforesaid system but also provides a system having a high degree of freedom which allows ease in acoustic manipulation of various musical notes. In this system, however, a system of sine wave orthogonal functions is used for Fourier inverse transformation. The system of sine wave orthogonal functions is a system of orthogonal functions in analog form and even if quantized and encoded, their values must be represented by a certain word length. This means the necessity of using a relatively large multiplier for multiplying the harmonic coefficient and each harmonic. For example, in case each harmonic amplitude and the harmonic coefficient are represented by n and m bits, a multiplication of n.times.m bits is required, which increases the number of elements forming the multiplier in parallel calculation and increases the calculation time in serial calculation.
In a device which has a large number of key switches, such as the keyboard of an electronic musical instrument, direct connection of the key switches to internal circuits for transferring information on the opening and closure of the switches to a desired one of the circuits inevitably involves an enormous amount of wiring, and hence is uneconomical. Also, the use of semiconductor integrated circuits or the like is difficult because of too large a number of pins.
In view of the above, there has recently been proposed a system which scans all key switches in a predetermined period of time and generates a pulse at the moment corresponding to a closed one of the key switches for each time sequence of the scanning, thereby to save the amount of wiring between the key switches and required circuits. For example, a key code multiplex system is usually employed in which information of a closed key switch detected by scanning the key switches on a time shared basis is sent in the form of a TDM (Time Division Modulation) or PCM (Pulse Code Modulation) signal. With this system however, the time for scanning all the key switches is fixed and this fixed scanning time is always required regardless of the number of key switches being closed, so that the scanning time is consumed wastefully in some cases.
The maximum number of keys which can be depressed simultaneously with both hands and a foot in the playing of an ordinary keyboard instrument is eleven. Assuming that blocks into which the keys are divided each correspond to one octave, it is impossible to depress the keys of two or more octaves with one hand, and accordingly the maximum number of block simultaneously occupied is five. Accordingly, the key switches are scanned for each block of them and if a key switch or switches are closed, the scanning is stopped for detecting them. Since the scanning is not stopped in the block having no key switch closed, one scanning time for obtaining switch information can be shortened.
The present applicants have proposed in the copending U.S. application Ser. No. 834,427 entitled "Electronic Musical Instrument" a key code generator of the variable frame method which shortens the scanning time in accordance with the abovesaid principle. Further, the present applicants have made a similar proposal in a different application recently filed. The outputs produced in the these systems are encoded, in which case the detection and assignment of depressed key data are very easy. On the other hand, in an electronic musical instrument of the type producing musical notes based on coefficient values of logic orthogonal functions described later, which has been proposed in our another application also recently filed, there is adopted, for simplification of the structure and processing, a method in which a plurality of non-encoded key data produced by simultaneous depression or releasing of a plurality of keys are assigned by an assignment unit and, thereafter, the addition of an envelope and a waveshape calculation are carried out in a plurality of systems. The detection and assignment of the depressed key data in this case present problems.